A paddle type flow detector utilizes a paddle which is inserted into a flow conduit, such as into a pipe. Paddle type flow detectors are designed, for example, to be mounted on a water pipe of sprinkler-type fire suppressant systems to activate an alarm when water flows in the pipe.
A paddle type flow detector is disclosed, for example, in U.S. Pat. Nos. 4,782,333 and 4,791,414, both assigned to the assignee hereof, herein incorporated by reference.
The known paddle type flow detector includes a switch mounted externally of the flow conduit and mechanically connected to the paddle. Movement of the paddle by force from a fluid flowing within the conduit causes a change in the state of the switch, generating a signal. The paddle is typically biased by a paddle spring into a base position corresponding to no flow in the conduit, and is movable against the spring bias into a "triggered" or "tripped" position by the flowing fluid.
Paddle type flow detectors have been used before within explosive environments, i.e., environments where explosive gases are likely to be present surrounding the detector. The Standard "UL 1203" sets forth criteria for explosion proof and dust-ignition-proof electrical equipment. In such equipment, sparks from opening and closing a flow indicating switch are isolated from the external environment by an enclosure or housing surrounding the switch and the housing prevents such sparks from propagating a flame or explosion outside of the housing.
However, it is possible that the switch enclosure or housing, over time, will permit the intrusion of explosive gases into the enclosure, particularly when the detector is located within an explosive environment for an extended period of time. It is possible then, that explosive gas and oxygen within the paddle switch housing can be ignited by the opening and closing of the switch contained therein. Such explosions can affect the sensitivity of the paddle spring which biases the paddle against the flowing fluid.
In the case of the paddle type detector used as a waterflow detector in a sprinkler system, the switch can be triggered more frequently than normally would be expected due to an activation of the fire suppressant system. Transient flows can occur in the sprinkler system pipes. For example, when a pump that maintains pressure in the system turns on, air trapped in the system will be compressed, which may cause water to temporarily surge past the flow detector location. Such temporary surges can trigger false alarms which are a nuisance. The common way to avoid such nuisance alarm is to provide a time delay between the beginning of the flow and in signaling an alarm. Not withstanding this time delay provision, the switch can be activated, i.e., change state between open and closed, many times more than actually needed to indicate a legitimate alarm. This would multiply the problem of spring exposure to explosions and the resultant effect on the sensitivity of the spring.
It would be desirable to provide a paddle-type flow detector which maintains its opening accuracy despite the possibility of explosions occurring within the switch housing of the detector.